Is it possible to create a fluid simulation of rain?
You can try the Fluid physics type in a particle emitter :


Add a particle system to a plane and configure it the way you want (number, start, max age...)
In the Physics panel, select Fluid.
Select the object you want the particles to run down on
Add a Collision physics modifier to this object (make sure it has enough faces), with particle damping Factor set to 1
hit play and:








Next, you just have to create a nice shader (this one is a blueish transparent shader, just for demonstration, very bad)

However, this type of simulation is not very realistic and some tweaks may be needed to suit your needs. It's just a track...